Recently, there has been growing interest in energy storage technologies. As energy storage technologies are extended to devices such as cellular phones, the intensive research and development of electrochemical devices has been made. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Among currently available secondary batteries, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
For this, the demand for an anode material having a high capacity is increasing. In order to meet the demand, Si-based materials having a large theoretical capacity have been used as an anode active material, however, Si deteriorates the life characteristics of batteries during repeated charging/discharging processes and causes severe thickness swelling, which adversely affects the performances and safety of the batteries. Accordingly, in order to maintain life characteristics and reduce thickness swelling, attempts have been made to use a silicon oxide (SiOx). However, the silicon oxide forms an irreversible phase due to the insertion of lithium, and thus has a low initial efficiency. To solve such a problem, the silicon oxide is first alloyed with lithium so that it contains lithium, thereby forming less of an irreversible phase material such as lithium oxides and lithium-metal oxides during initial charging and discharging processes, and eventually improving the initial efficiency of an anode active material. The lithium source, which is used to first alloy a silicon oxide with lithium, may be divided into a lithium source using metallic lithium, a lithium salt having no oxygen, and an oxygen-containing lithium salt.
Among these, the metallic lithium has a great reactivity with water and may be dangerously apt to ignite, and may also react with carbon dioxide to produce lithium carbonate. Also, most lithium salts not containing oxygen are formed by an ionic bond, and thus the reaction of lithium salts and silicon oxides is very restricted. Therefore, it is preferable to use lithium salts containing oxygen.
However, while silicon oxides react with lithium salts containing oxygen, oxygen present in the lithium salts react with the silicon oxides, thereby making it hard to control the amount of oxygen in the silicon oxides. Also, the remaining unreacted lithium sources and by-products of the reaction between the lithium sources and the silicon oxide may lead to the gelation of an anode active material-containing slurry in the preparation of an electrode.